Recycling of iodine in fore-arc areas: evidence from the iodine brines in Chiba, Japan

The distribution of iodine in the Earth’s crust is dominated by its accumulation in marine sediments. If fluxes between terrestrial and marine compartments are considered, however, a significant imbalance exists between known sources and sinks of iodine. We present here evidence from the fore-arc area near Chiba, Japan, the world’s largest brine-iodine producing area, that iodine is mobilized from marine sediments during the early stages of subduction. Based on detailed chemical analyses of 22 brines and 129I dating of 13 of these samples collected from the Kazusa Formation, we show that iodine in these fluids is derived from organic-rich marine sediments with a minimum age of 50 Myr. Geochemical characteristics of the brines and the age of the iodine indicate that the iodine enrichment is caused by mobilization from subducting marine sediments and not by derivation from the host formation (age 1–2 Myr). The direct return of iodine from marine sediments into the oceans during the subduction of oceanic plates could provide the missing link in the iodine cycle and be an important pathway also in the marine cycle of carbon.